1. Field of the Invention
This invention relates to mirrors and, more particularly, to a method for altering the coefficient of thermal expansion (CTE) of a mirror.
2. Description of Related Art
Small but nonetheless significant variations in the coefficients of thermal expansion (CTEs) between the front and back faceplates of mirrors used in a segmented aperture system can have significant effects on the optical quality of the image. Upon the application of a temperature change these mirrors will bend and the probability of them all bending identically, even for a uniform temperature change, is vanishingly small considering the precision required of these elements. Specifically the radius of curvature of these mirror segments need to be made and maintained to tolerance level approaching only several parts per million. From the following relationship ##EQU1## where .DELTA..alpha. is the difference in CTE's between the two faceplates and .DELTA.T is the bulk temperature change, a 5 ppm radius of curvature precision (.DELTA.R/R) would demand that .DELTA..alpha. be 5 part per billion (ppb) for a .DELTA.T of 5.degree. F. for a segment whose radius of curvature, R, was 25 m. The effect of CTE mismatches can be compensated for by mechanical actuators for bending the mirror back into shape, but this adds undue complexity for certain applications. On the other hand, with a sufficient inventory of glass, such as Ultra Low Expansion (ULE) glass from Corning Glass Co., from which to choose, a CTE matching could be accomplished for as many segments as are needed for any given system, although this is a difficult task in terms of cost, schedule, etc. Mitigation of these difficulties would be desirable. In other words, a better method is needed to compensate for these variations to facilitate meeting the sometimes stringent segment-to-segment matching requirements that are often necessary. This would most desirably be accomplished after the segments are fabricated and their actual coefficients of thermal expansion determined by optical measurement in the presence of a known temperature distribution.